There are a number of electrochemical analytical techniques which may be employed for determining the presence and/or concentrations of various chemical species. Electroanalytical techniques have enjoyed particular success in the measurement of the concentrations or activities of gaseous or ionic species dissolved in liquids, and have been applied inter alia, to the analysis of blood and other biological fluids. Polarographic analyses are particularly well-suited for the determination of a variety of dissolved species and in general involve the immersion of a pair of electrodes into a sample containing the analyte. One of the electrodes is termed the working electrode and a second is a reference electrode. A preselected potential is applied to the working electrode relative to the reference electrode and this potential causes a change in the oxidation state of a given species of interest at the electrode/sample interface and a transfer of electrons thereacross. The transfer of electrons results in a flow of current which is proportional to the concentration or activity of the target species at the surface of the working electrode. By measuring the amount of current flowing between the two electrodes, the concentration of the target species may be determined accurately.
Polarographic methods may be advantageously employed for the determination of the concentration of dissolved oxygen in a fluid sample. In such instance, the measuring electrode is generally fabricated from a noble metal such as gold or platinum, selected for its nonreactivity with the oxygen. The reference electrode is typically metal, with a metal halide coating formed thereupon, such as silver with a surface coating of silver chloride. It could also be a composite material containing a metal halide, such as a silver chloride pellet. Application of a negative potential to the working electrode (e.g., a Pt electrode) causes a flow of electrons from the electrode to the oxygen atoms which results in the reduction of oxygen to H.sub.2 O.sub.2 and/or OH at the electrode surface. This flow of electrons or current flow can be measured in an external circuit interconnecting the two electrodes, and such flow is proportional to the rate of oxygen diffusion to the working electrode, which in turn is proportional to the concentration of oxygen. By appropriate choice of electrode material and applied potential, analyses of other species including other dissolved gases, neutral species and ions may be similarly accomplished. In some instances, the electrode material itself is directly reactive with the dissolved species to effect a transfer of electrons in the absence of an applied external potential; however, the principles generally remain the same and this potential (or current flow resultant therefrom) is measured.
In most practical applications, the afore-described electrodes are both separated from the sample fluid by means of a membrane preferably having a selective permeability. The membrane screens out interfering species present in the sample and serves to provide a fixed analytical environment in which the electrodes operate. U.S. patent application Ser. No. 148,155, now U.S. Pat. No. 4,871,439, which is assigned to the assignee of the present invention discloses one such prior art electrode assembly. Referring now to FIG. 1, there is shown a prior art polarographic electrode assembly including a flow channel 10 configured to carry a stream of sample fluid therethrough. The electrode assembly further includes a working electrode 12 formed from a length of platinum wire and a reference electrode 14 comprised of a length of silver wire having a silver chloride coating 11 upon at least the active face thereof. The working electrode 12 and reference electrode 14 are in electrical communication via a body of electrolyte material 11 which is separated from the fluid in the fluid flow channel 10 by a membrane 19. The membrane 19 is a hydrophobic membrane, typically formed of material such as poly(vinyl chloride), and having a permeability to oxygen and water.
While electrode assemblies of this type have been found to provide excellent results in terms of accuracy and reliability it has been recognized that many advantages would attend upon the placement of the reference electrode outside of the membrane in the sample fluid. Fabrication of the electrode assembly would be simplified insofar as the need for the common electrolyte layer (17 in FIG. 1) establishing a conductive bridge between the two electrodes would be eliminated; hence a smaller, thinner electrolyte layer could be readily utilized. It has been found that problems arise in the preparation and use of the relatively thick electrolyte layer necessitated by the presence of the reference electrode beneath the membrane. The hydrophobic membranes are prone to manifest openings therethrough when deposited atop the irregular geometry of the thick electrolyte layers. Hence, thicker, difficult to prepare membranes must be employed. Additionally, the thick membranes slow the response time of the electrode. While it would be advantageous to place the reference electrode outside the membrane, many problems arising in conjunction with such a configuration have heretofore prevented use of structures of this type.
It has been found that simply placing the reference electrode outside of a hydrophobic membrane causes problems in the operation of the sensor, because of the low electrical conductivity of the hydrophobic membrane due to the fact that ions pass therethrough very slowly. Sensors utilizing more permeable hydrophilic membrane materials, such as cellulose acetate butyrate, enjoy only limited success insofar as such membranes are not very selective in their permeability and hence allow many hydrophilic interfering ions and neutral species therethrough. For example, if an electrode assembly having a hydrophilic membrane were utilized for an analysis of dissolved oxygen in a blood sample, erroneous results could occur owing to interference from ions such as Cu.z or other reducible species which could readily diffuse through the hydrophilic membrane.
As a result of such problems the prior art has heretofore generally disposed working and reference electrodes in polarographic sensors beneath or behind a common membrane. For example U.S. Pat. No. Re. 31,299 discloses an analytical electrode assembly for measuring oxygen concentrations and including working and reference electrodes and electrolyte disposed beneath a common hydrophobic membrane. In addition to the common hydrophobic membrane, the '299 apparatus includes separate ion-selective membrane coatings on the active surfaces of the reference and measuring electrodes. Similarly, U.S. Pat. No. 4,685,465 shows another prior art oxygen sensor having both measuring and reference electrodes disposed behind a single, oxygen permeable membrane.
In an attempt to avoid problems caused by the low electrical conductivity of hydrophobic membranes, various attempts have been made to utilize hydrophilic membranes for electrochemical analyses; however, as mentioned previously, such membranes are not selective in regard to species transmitted therethrough. Pat. No. 4,672,970 describes a measuring electrode having a hydrophilic membrane disposed upon a face thereof and teaches that such hydrophilic materials are superior to, and preferred over hydrophobic materials; however, no attempt is made to deal with problems engendered by the lack of selectivity of such membranes.
It has been found in accord with the present invention that electrode sensor assemblies may be fabricated having working electrodes separated from reference electrodes by hydrophobic membranes provided that the membranes are selected and fabricated to have sufficient ionic conductivity. It has been found in accord with the present invention that hydrophobic membranes may be rendered sufficiently conductive for use in polarographic electrode assemblies by appropriately doping or otherwise modifying the polymeric materials. It has further been found that the membrane is "sufficiently conductive" if its electrical resistance is less than the electrical resistance presented by the remainder of the measuring apparatus. That is to say that the membrane itself should not present the primary limiting step to electrical conduction and hence measurement.
It has also been found that merely rendering the membrane conductive is not sufficient to ensure accuracy in a great many instances. Problems occur because an electrical potential may develop across the hydrophobic membrane owing to different concentrations of ionic species present on either side thereof. This potential adds to the applied working electrode potential and, since it is generally of an unknown magnitude, represents a potential source of error. It has been found, in accord with the principles of the present invention, that this potential can be made to be relatively constant if the membrane is made permeable to a given ionic species known to be present at relatively constant concentrations in all the analyte fluids which will contact the electrode assembly.
It will therefore be appreciated that the present invention solves problems which have heretofore restricted the accuracy, size and fabrication ease of polarographic type electrode assemblies. By the use of the present invention, electrode assemblies may be fabricated having the reference electrodes separated from the measuring electrodes by a selectively permeable, hydrophobic membrane having reasonably high electrical conductivity and manifesting a stable electrochemical potential thereacross in analyte fluids having various ionic compositions therein. These and other advantages of the present invention will be readily apparent from the drawings, discussion, description and claims which are a part hereof.